In German Patent No. 30 12 186 a time measurement method is disclosed in which the time distance of two repeatable time-significant signals is to be determined as accurately as possible. The measurement value concerned is obtained not directly, but by calculation from three individual measurement values, a so-called coarse measurement value and two so-called precision measurement values. In that case, the coarse measurement value is obtained by counting the waveform periods of a time base signal, which occur between both the time-significant signals. The time base signal is delivered by a free-running, quartz-controlled oscillator, i.e. by an oscillator which is not at all correlated or synchronised with both of the time-significant signals. Both the precision or fine measurement values, which each represent the time distance of the earlier or later time-significant signal from the beginning (or end) of a succeeding, for example of the beginning of the next waveform period of the time base signal, are measured in analog manner with the aid of one and the same time-to-amplitude converter circuit and, after an analog-to-digital conversion has taken place, combined correctly in sign with the coarse measurement value for the calculation of the overall measurement value of interest. In the normal case, this means that one of both the fine measurement values is added to the coarse measurement value and the other is subtracted from the coarse measurement value so that altogether the difference between the two fine measurement values, enters into the measurement result actually of interest. In that case, the rising, or the falling edges of a rectangularly shaped time base signal or corresponding zero transitions of a sinusoidal time base signal can serve as reference points marking the beginning or the end of a waveform period.
A typical example of application for this known time measurement method is represented for example by distance measurement methods on the principle of the transit time measurement of a light pulse, in which the light pulse emitted by a transmitting light source is divided up within the measuring device into a measurement light pulse and a reference light pulse, wherein the first is emitted through the optical transmitting system towards the target object, is reflected from there to the measuring device and forwarded in the measuring device to a light receiver which has previously received the reference light pulse through a short reference light path, internal to the device, of known length. Both light pulses initiate in the receiving channel following the light receiver two signals which represent the time-significant signals of the above mentioned time measurement method and the time distance of which is a measure of the distance of the target object. Because of the normally different length of the reference light path and of the measurement light path, each of both these initially simultaneously generated individual signals is subject to an own time of travel which differs from that of the other signal. As long as however the distance between the target object and the distance measuring device does not change, the times of travel of both these signals also remain unchanged, so that repeatedly both kinds of light pulses can be generated for repeatedly measuring the difference of their transit times.
Another possibility of application of the above mentioned time measurement method is the measurement of the phase displacement between two periodic signals of the same frequency. If one for example assumes that periodic rectangular signals are concerned in the case, then two rising edges, following one upon the other within a period length, of the one and of the other periodic signal are the repeatable individual signals and the phase displacement between both the periodic signals can be measured in that one measures the time distances of these repeatable individual signals from a following defined, for example the next rising edge of a time base signal possessing the same frequency, wherein the difference of both these time distances then again enters into the measurement result of interest. The main difference from the time measurement method known from German Patent No. 30 12 186 is that only both the so-called fine measurement values, but not a coarse measurement value, are determined. The sum of the different delays, or "transit times", which both the repeatable individual signals are subjected to before the time measurement, is equal to the phase displacement to be measured. In that case, both these "transit times" are not generated thereby, that both the periodic signals have been generated simultaneously, i.e. without phase displacement, and then traversed paths of different length. Instead thereof, the different delays can have been imparted to both the periodic signals also from the start, i.e. already during their generation.
Beyond that, still further cases of application for the above named time measurement methods are feasible, which however all have in common that for two different types of pulse-like signals of short duration, the respective transit time has to be measured, whereby not the two transit times by themselves, but their difference is of interest. Furthermore the invention is only applicable in such cases, in which a signal of each type can be repeated several times whereby the respective transit time remains unchanged. For sake of simplicity in the following the signals of the first type are called "measuring pulse signals" since they can be used for example to travel along a distance of interest, whereas the signals of the second type are called "reference pulse signals" since they are mainly generated to establish a reference transit time which is to be subtracted from the transit time of the measuring pulse signals in order to eliminate measurement errors or in order to take into account a reference value. In any case, the transit time measurements for both types of signals comprise a so-called precision or fine time measurement in which the time distance of the respective pulse signal from a defined succeeding or preceding rising (or falling) edge of a rectangular time base signal or a succeeding or preceding defined zero transition of a sinusoidal time base signal, is to be measured and the differences of precision time measurement values for measuring pulse signals on the one hand and for reference pulse signals on the other hand are to be processed for the obtaining of a further measurement value. It is in that case presupposed that the difference between the time distances of the measuring pulse signals and the time distances of the reference pulse signals from the corresponding point of the time base signal remain unchanged during the repetitions of these individual signals because of the constancy of the respective transit times during these repetitions. The time distances are measured by one and the same time-to-amplitude converter circuit, wherein, when the measurement values to be subtracted one from the other are obtained sufficiently rapidly one behind the other, not only long term and medium term drift phenomena, but also short term fluctuations are eliminated because of the difference formation. It is in that case, however, required for the elimination of fluctuation and drift phenomena constantly to recalibrate the time-to-amplitude converter circuit with the aid of a standard of comparison. For this purpose the waveform period length of the time base signal is well suited when the time base signal is generated with the aid of a quartz-controlled oscillator. However, only individual points of the characteristic curve of the time-to-amplitude converter circuit can be determined by comparison with said time base signal, which points lie spaced one from the other by exactly one period length of the time base signal. The calibrated ranges of the charateristic curve disposed between these calibration points are of particular importance for the reason that the time distances to be measured for determining the transit times of the measuring and reference pulse signals usually are not an integral multiple of the period length of the time base signal. This means that the corresponding measurement values, i.e. the measured amplitudes of the output signal of the time-to-amplitude converter circuit normally lie somewhere between the amplitude values obtained at the calibration measurements. To obtain a time value from a measured amplitude value, therefore, the distance or distances between the spaced-apart calibration values have to be bridged over in the simplest case by a linear interpolation.
This causes a problem since the characteristic curve of the time-to-amplitude converter circuit can have such large non-linearities between the calibration points that impermissibly large errors can occur at least when a very high accuracy is demanded. The reason for these large errors is that the measurement values for the measuring pulse signals on the one hand and for the reference pulse signals on the other hand generally lies at different points of the characteristic curve of the time-to-amplitude converter circuit so that they contain error components caused by the non-linearity of the characteristic curve which are so different that they do not sufficiently cancel each other out when the one measurement value is subtracted from the other.
A possibility for the reduction of these errors consists in repeating the measuring pulse signals as well as the reference pulse signals so many times and completely uncorrelated with the time base signal that the error in the mean value resulting during a following mean value formation falls below the demanded limit of accuracy according to the root law of the error theory. In the case of very large non-linearities in the characteristic curve of the time-to-amplitude converter circuit or in the case of a very high demanded measurement accuracy, an extra-ordinarily large number of repetitions of the individual measuring and reference pulse signals can however be required for this, so that the time which is required for the obtaining of a correspondingly exact measurement and result, becomes very long. Particularly in the case of distance measurement methods, in which the distance of a very rapidly moving object shall be measured, such a long measurement time is not available, since the difference in the time distances of the individual measuring and reference pulse signals from the defined points of the time base signal must not change during this measurement time.
An objective of the invention is to provide a method of the foregoing kind that, in spite of the presence of unknown non-linearities in the characteristic curve of the time-to-amplitude converter circuit, results in measurement values of very high accuracy which can be obtained within a very short measurement time.